The present invention relates generally to cookware for the home and commercial market, including cooking griddles, sauce pans, stock pots, woks and fry pans and, more particularly, to a form of composite cookware having a hollow, gas-filled cavity between the bottom heat surface and the top cook surface.
An important consumer concern for selection of home cookware is to provide uniform heating across the cooking surface. Heat sources such as gas or electric ranges generally provide non-uniform heating, so the cookware needs to correct this non-uniformity. Uniform heat distribution helps to avoid burning food and also helps eliminate undercooking. For this, the use of a metal vessel having a high thermal conductivity coefficient is helpful. Metals such as aluminum, copper and silver have a high thermal conductivity coefficient; however the use of aluminum or copper in contact with food is shunned by many consumers due to perceived potential health issues. The use of silver as a cooking surface is impractical because of its high cost and because it is prone to tarnishing. Stainless steel is the preferred metal to use with regard to taste, stain resistance and fewer perceived potential health issues; however it has poor thermal conductivity. That property makes the use of stainless steel cookery impractical for high performance cookware.
To meet the conflicting requirements of good thermal uniformity, stain resistance and health concerns, high performance cookery has been developed that uses a laminated construction consisting of multiple layers of metal. In these, embedded layers of aluminum, copper, or silver are sandwiched inside the cookware shell to aid in heat distribution. These embedded metal layers do not contact the food, but help to compensate for the poor heat distribution of stainless steel used for the cooking surface. For esthetic reasons, stainless steel may also be used on the heat surface of the cookware that is in contact with the heat source.
Examples of this laminate construction in the prior art is evidenced by U.S. Pat. No. 6,109,504, which describes the use of a thick, thermally conductive core of copper metal that is bonded to a cook surface of stainless steel. Similarly, U.S. Patent Application Publication US2008/0241582 describes the use of multi-ply cookware having layers of copper, silver and aluminum that are embedded between a top, cook surface of stainless steel and an esthetic, outer shell of stainless steel that is in contact with the heat source. As yet another example, is US Patent Application Publication US2006/0283844, which describes the use of laser welding to seal the rim of multi-layer cookware that comprises an inside sheet of copper for uniform heat distribution. That publication also teaches that is important to heat the cookware prior to brazing and sealing in order to help remove any trapped gases or moisture before this sealing is done.
These approaches improve upon the problem of uniform heat distribution for the cookware but also result in a new consumer concern: weight of the cookware. The use of multiple layers of metal in the cookware adds to the weight of the product. The result is that high performance cookware is often heavy. The weight of the product becomes a hindrance to its use, its storage and the ease with which the cookware can be cleaned. Heavy cookware is especially difficult for the elderly and many women to use. As an attempt to the reduce the weight of laminated, multi-layer cookware, US Patent Applications US2011/0041708 and US2009/0152276 describe the use of high conductivity, low density carbon foam used for heat distribution in place of heavy transition metal layers that are embedded into cookware. These patent applications also describe the use a vacuum pump to help promote direct contact (thermal conduction) between the carbon foam core and the upper and lower layers of stainless steel. In some cases, a vacuum pump is attached to the cookware during use. The use of a vacuum pump as part of a griddle having multi-layer metal construction is described in U.S. Patent Application US2011/0162535. The use of a permanently sealed, interior vacuum condition for multi-layer cookware has also been described in that publication.
U.S. Pat. No. 4,541,411 describes the use of a thermally anisotropic graphite layer used between metal layers for improved heat distribution in multi-ply cookware. The low density of the graphite core helps to reduce the weight of the cookware. Graphite is also good for heat distribution because it exhibits “anistropic” thermal conductivity: i.e., graphite has different values of thermal conductivity when measured along different crystalline axes. In the '411 patent, the thermal conductivity along the plane of the cookware's flat surface exceeds the thermal conductivity in the axis perpendicular to the flat surface, thereby helping to eliminate hot spots. Ordinary (and inexpensive) graphite has only limited such anisotropy because it is amorphous. Pyrolytic Graphite Sheet (PGS), is an exceptionally good choice for this purpose because its anisotropy is extremely high (>10×) and its thermal conduction in the flat plane is twice that of copper (which has only isotropic thermal conductivity characteristics). PGS is available in sheet thickness of about 0.1 mm. It is also heat resistant to about 500 C, which is ideal for cookware. The extremely high cost of PGS, however, makes its use for multi-layer composite cookware prohibitively expensive. However, small pieces (such as static vanes in the present invention as described later) could be economically viable. Information on pyrolytic graphite sheet can be obtained at http://www.panasonic.com/industrial/components/pdf/pgs_info—0105.pdf
Similarly, the use of graphite fibers for anisotropic heat conduction in cookware and glass vessels is described in U.S. Pat. No. 4,265,968. In this invention, shaped carbon fibers are incorporated into a glass matrix. Carbon nanotubes also provide anistropic thermal conductivity characteristics, as described by S. Berber et al., Phys. Rev. Lett, 84(20), pp 4613-4616, (2000), see http://www.pa.msu.edu/cmp/csc/eprint/DT130.pdf. Carbon nanotubes could be used for improved heat distribution in the present invention, once these become widely available.
U.S. Pat. No. 6,305,272 purports to describe the use of a “vacuum” container that envelopes the sides and bottom of a stock pot cooker. In this patent, an unidentified “action fluid” partially fills (to a volume level of 20%) the “vacuum space” to aid in heat transfer. However, the cooker of U.S. Pat. No. 6,305,272 is highly practical and dangerous if it can function at all as described. The “action fluid” is said to evaporate when heated, and that this evaporation leads to a “phase change heat transfer effect”. However, the conversion of a liquid to gas leads to a massive volumetric expansion, which would cause very high pressures to develop in the “vacuum space”. This leads to a very significant risk of explosion. In addition, the “phase change heat transfer effect” would not promote rapid heat transfer, but the opposite, as much of the applied heat would go into effecting the phase change rather than being transferred to the contents of the cooker. The device described in U.S. Pat. No. 6,305,272 only can be understood, if at all, by supposing that the “action fluid” is a low vapor pressure liquid with a high boiling point that does not in fact volatilize. Curiously, heating of the action fluid 14 in FIG. 4 of the '272 patent is accomplished by heat transfer through base body 12 in FIG. 4 of the '272 patent, which is a vacuum space that is not filled with any fluid. Heat transfer through any vacuum cavity would be expected to be very poor, so it is not clear how this device actually operates.